Factors in the social environment, such as psychosocial stimulation and stress from interactions with conspecifics, may influence the timing of sexual maturation. The degree of sociality of a species may affect the responsiveness of females to social signals or the mechanism by which social cues influence reproduction. The long-term goal of this research program is to examine the mechanisms that affect the timing of reproduction. Understanding the nature of the social cues that may influence reproduction and sexual development is central to uncovering the physiological and neural mechanisms, as well as the evolutionary basis, of reproductive timing in mammals. There are three specific goals of these experiments: (1) to determine whether or not social cues can suppress or delay reproduction in two species of rodents that differ in the degree of sociality, (2) if females in both species exhibit reproductive suppression, then to examine the cues involved in mediating suppression and, (3) to test the effects of suppressive cues under natural conditions. Two rodent species will be compared in these experiments: the pine vole, a cooperative breeder, and the meadow vole, which tends to be less social during the breeding season. In a number of cooperatively breeding species, only one female typically reproduces although non-reproductive group members also provide care to young. Delayed reproduction has also been reported in noncooperatively breeding females, at least during periods of high population density. Lack of or delayed reproduction seems extremely costly to individuals due to losses in direct fitness (Brown 1987). Initial experiments (Exp 1 and 2) will determine the source of suppression of younger females. The relative effectiveness of chemical versus behavioral cues in suppression of reproduction will be examined (Exp 1 and 2). Variations in the amount of contact with a potential mate and suppressive cues will be examined to determine the likelihood of suppression of reproduction versus lack of sufficient contact with males as an explanation of suppression in the wild (Exp 3). Finally, the frequency with which chemical cues need to be presented will be determined prior to tests in a natural setting (Exp 4). If suppression of reproduction occurs in response to chemical cues in the laboratory, field experiments will be conducted to assess the effectiveness of chemical cues on reproduction in semi-natural settings (Exp 5 and 6). Breeding females will be removed and then chemical cues presented to determine if chemical cues alone can suppress reproduction in a field situation. Finally, a similar study using chemical cues will be conducted in an orchard setting with pine voles (Exp 7). The results of these experiments will provide valuable insights into the mechanisms and selection pressures responsible for the evolution and/or maintenance of delayed reproduction in mammalian species that differ in the degree of sociality.